A conventional amplifier comprises, as shown in FIG. 1 for example, an input stage 10, a voltage amplified stage 20 and a SEPP (Single Ended Push-Pull) output stage 30.
The input stage 10 includes a pair of FETs 11 and 12 of monolithic IC, an input terminal 13 connected to the gate of FET 11, an input resistor 14 connected between input terminal 13 and the ground, a resistor 15 connected between the common source of FETs 11 and 12 and a negative power supply -Vcc, and resistors 16 and 17 which are connected between a positive power supply +Vcc and the respective drain of FETs 11 and 12. The gate of FET 12 to the ground and the output terminal are connected negative feedback resistors 18 and 19 which determine the gain or amplification factor of the amplifier.
The voltage amplified stage 20 includes a PNP transistor 21 having a base connected to the drain of FET 11, a PNP transistor 22 having a base connected to the drain of FET 12, a resistor 23 connected between the common emitter of PNP transistors 21 and 22 and the positive power supply +Vcc, an NPN transistor 24 having a collector and a base which are connected to the collector of PNP transistor 22, and an NPN transistor 25 having a base connected to the collector of PNP transistor 22. These NPN transistors 24 and 25 have emitters each connected to the negative power supply -Vcc. The absolute value of the collector current of PNP transistor 22 is substantially equal to that of NPN transistor 25. The collector of PNP transistor 22 provides a first relay output terminal I.sub.1 while the collector of NPN transistor 25 provides a second relay output terminal I.sub.2.
The SEPP output stage 30 has Darlington-connected NPN transistors 31 and 32 each having a collector connected to the positive power supply +Vcc, Darlington-connected PNP transistors 33 and 34 each having a collector connected to the negative power supply -Vcc, an output terminal 35, a first resistor 36 directly connected to output terminal 35 and connected to the emitter of NPN transistor 32, and a second resistor 37 directly connected to output terminal 35 and connected to the emitter of PNP transistor 34. The junction between the emitter of NPN transistor 31 and the base of NPN transistor 32 is connected to output terminal 35 via resistor 38, while the junction between the emitter of PNP transistor 33 and the base of PNP transistor 34 is connected to output terminal 35 via resistor 39. The constant-voltage bias (CVB) circuit 26 is connected between first relay output terminal I.sub.1 connected to the base (first control end) of NPN transistor 31 and second relay output terminal I.sub.2 connected to the base (second control end) of PNP transistor 33.
The bias circuit 26 includes a temperature-compensation NPN transistor 27 having a collector connected to the base of NPN transistor 31 and an emitter connected to the base of PNP transistor 32, a variable resistor 28 connected between the bases of NPN transistors 31 and 27, and a fixed resistor 29 connected between the bases of NPN transistor 27 and PNP transistor 33. The idle current flowing between collectors of NPN transistor 32 and PNP transistor 34 is regulated to class AB through adjustment of variable resistor 28. The voltage between the collector and the emitter of NPN transistor 27 has a value which is obtained by adding the resistances of variable resistor 28 and that of the fixed resistor 29 to produce a sum value, dividing the sum value by that of fixed resistor 29, and multiplying the quotient with the base-emitter voltage V.sub.be across NPN transistor 27. In the conventional CVB circuit 26, since the voltage V.sub.be is substantially constant, the bias voltage between the first and second relay output terminals is maintained constant.
In the operation of this class AB amplifier, for example, when NPN transistor 31 and 32 are driven to the plus voltage side, PNP transistor 33 and 34 are also driven to the plus voltage side through the CVB circuit. More specifically an increase in the base currents of NPN transistors 31 and 32 causes the collector currents to increase correspondingly, as well as the respective base to emitter voltage V.sub.be of NPN transistors 31 and 32. The voltage across first resistor 36 also increase as a result of the increase in the collector current. In consequence, since the voltage between the collector and the emitter of NPN transistor 27 is substantially constant, the voltage applied between the emitters and the bases of PNP transistors 33 and 34 are reduced to turn off PNP transistors 33 and 34.
In general, bipolar transistors such as NPN or PNP transistors require a certain transition time for transiting from the "off" state to the "on" state, in order to charge up the holes or the electron carriers. Thus the charging time causes an unfavourable effect on the characteristics of the amplifier.
The PNP transistors 33 and 34 require, when driven to the minus voltage side from the completely "off" state, carrier currents for charging electrons in the transistors in addition to the base current components proportional to the ordinary collector currents. The carrier currents adversely affect the non-feedback characteristics of the amplifier. When the amplifier is connected to an inductive load such as a loudspeaker, the carrier currents cause unfavourable effects on the output such as the quality of the sound, due to time lag in the switching between "on" and "off" states of NPN transistors 31 and 32 and PNP transistors 33 and 34. In consequence, the conventional amplifier of class AB has suffered from a problem in that the quality of the sound is impaired to a degree which is not measurable by ordinary measuring instrument, due to the use of the CVB circuit.